1. Technical Field
This invention relates to the art of radio broadcasting and, more particularly, to an AM transmitter provided with feed-forward correction to compensate for source impedance.
2. Description of the Prior Art
The U.S. Patents to H. I. Swanson U.S. Pat. Nos. 4,580,111 and 4,949,050 disclose an amplitude modulator for use in AM radio broadcasting and wherein the modulator serves to generate an amplitude modulated signal by selectively turning on and off a plurality of RF amplifiers in digital manner to produce amplitude modulation. Each of the RF amplifiers provides an output voltage across the primary winding of a transformer. The secondary windings of the transformers are connected together in series in a series combiner. Output current flows in the secondary windings to supply a load, such as an antenna, for broadcasting an RF signal.
In such a system as noted above, a power supply provides DC operating voltage to the plurality of RF amplifiers. Since the magnitude of the power supply may vary somewhat over a period of time, it is known to provide power supply compensation by means of a corrector that supplies the reciprocal value of the power supply voltage to the input signal prior to amplification. This compensates for any variations in the value of the power supply voltage. However, this feed-forward power supply variation compensation overlooks the error caused by the source impedance as presented to the output load by the amplifiers. This is an error that results in non-linear amplification of the input signal resulting in distortion.
In accordance with the present invention an AM transmitter is provided having feed-forward correction to compensate for source impedance. The transmitter includes a power supply source for providing a DC operating voltage. N RF amplifiers are provided for amplifying an input signal. The amplifiers are connected to the power supply source with each amplifier having an output circuit that, when the associated amplifier is on, supplies energy from the source to provide an output voltage at a load. Each amplifier output circuit exhibits a source impedance that degrades the value of the output voltage. A source impedance corrector varies the value of the input signal prior to amplification by the amplifiers in order to compensate for any variations in the value of the output voltage caused by the source impedances.